Turbofan engines are frequently employed in aviation. In order for a turbofan engine to be effective in an aviation application, it is typically placed close to other critical portions of an aircraft. For example, a turbofan engine may be mounted on a wing thus placing the turbofan engine in close proximity with both the wing and the fuselage of the aircraft. Therefore, operation of such a turbofan engine must meet certain safety standards.
Safety standards and precautions for turbofan engines are important during all phases of operation, including start-up, shutdown, normal operation, and failure. Safety may be of particular concern during failure of the engine, especially when failure involves the fan itself. One type of failure condition is known as “fan blade off.” “Fan blade off” refers to separation of a fan blade from the turbofan shaft. During a fan blade off event, a case housing the turbofan engine is expected to prevent the fan blade from travelling along a path that damages the wing or fuselage.
Referring now to the prior art design shown in FIG. 2, a metal hardwall case 40 may be made of titanium and include a metal/titanium barrel 28. A typical titanium hardwall case 40 design may include a metal containment hook feature 30 machined as part of the metal fan case barrel 28. The integral metal hook feature 30 is manufactured at the same time as the metal fan case barrel and is integral to said metal fan case barrel 28, as seen in FIG. 2. A fan track liner 34, traditionally made of metal honeycomb, lines the metal fan case. The fan blade(s) 36 are secured to a shaft during normal operation. Upon separation from the shaft, the fan blade 36 strikes the metal hardwall case 40.
During the fan blade off event, the fan blade travels in both a radial/circumferential and possibly axial direction away from the turbofan shaft. This movement results in the fan blade 36 moving out towards the metal fan case barrel 28 and forward in relation to the turbofan engine. It is an objective of the metal hardwall case 40 to prevent the fan blade 36 from escaping the metal fan case 40. The fan blade 36 escaping from the fan case 40 is a safety hazard and may result in damage to the fuselage or wing caused by an impact from the escaped fan blade 36.
As an alternative to the integral hook machined into the metal fan case barrel, a composite fan case may be elongated and include at the termination of the elongated case a debris fence that assists in containing a separated fan blade during a fan blade off event. Elongating the fan case requires additional material and additional weight. A need exists for a turbofan engine casing that uses composite materials, provides containment of a separated fan blade, and achieves weight reduction.